A NEW IMAGE of Messier 55 from the European Southern Observatory’s (ESO) VISTA infrared survey telescope shows tens of thousands of stars crowded together like a swarm of bees. Besides being packed into a relatively small space, these stars are also among the oldest in the Universe. Astronomers study Messier 55 and other ancient objects like it, called globular star clusters, to learn how galaxies evolve and stars age.
Globular clusters are held together in a tight spherical shape by gravity. In Messier 55, the stars certainly do keep close company—approximately one hundred thousand stars are packed within a sphere with a diameter of only about 25 times the distance between the Sun and the nearest star system, Alpha Centauri.
About 160 globular clusters have been spotted encircling our galaxy, the Milky Way, mostly toward its bulging centre. The largest galaxies can have thousands of these rich collections of stars in orbit around them.
Observations of globular clusters’ stars reveal that they originated around the same time—more than 10 billion years ago—and from the same cloud of gas. As this formative period was just a few billion years after the Big Bang, nearly all of the gas on hand was the simplest, lightest and most common in the cosmos—hydrogen, along with some helium and much smaller amounts of heavier chemical elements such as oxygen and nitrogen.
Being made mostly from hydrogen distinguishes globular cluster residents from stars born in later eras, like our Sun, that are infused with heavier elements formed in earlier generations of stars. The Sun lit up some 4.6 billion years ago, making it only about half as old as the elderly stars in most globular clusters.
The new image was obtained in infrared light by the 4.1-metre Visible and Infrared Survey Telescope for Astronomy (VISTA) at ESO’s Paranal Observatory in northern Chile.
As well as the stars of Messier 55, this VISTA image also records many galaxies lying far beyond the cluster. A particularly prominent edge-on spiral galaxy appears like a thin, red smudge to the upper right of the centre of the picture.
Adapted from information issued by ESO / J. Emerson / VISTA. Acknowledgment: Cambridge Astronomical Survey Unit.
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